We are going to start this model in equilibrium. We will set number of houses required to 5000, which is the initial value for houses completed. Because of this, gap in houses, will be 0 so that planning is equal to replacement houses which, in turn is equal to demolishing. Because of the way we initialized the other levels (for example Planned Houses is initialized equal to planning * time to plan to build) each of these is already in equilibrium. We should therefore have a model that will simulate without any values changing.
While it is important to check to be sure that our understanding is correct and the model will simulation without anything changing, we also want to see more model behavior. Thus, instead of using just 5000 for number of houses requiredwe want to hold it at 5000 for a time (say ten months) and then increase it (say to 5050). To do this we use the equation:
number of houses required = 5000 + STEP ( 50, 10)
Units: house
The STEP function takes two arguments, height and start time, which are enclosed in parentheses. It takes on a value of 0 until start time is reached and from then on takes on value height. This function is a particularly good input for a model because it is a simple input change that generates a broad behavior response. Other functions that are useful for "exciting" or "disturbing" a model in this way include PULSE and RAMP.
To add the above equation, open the Equation Editor on number of houses.
| Ø | Type in the number 5000, then the plus sign. |
| Ø | Click on the Functions tab then scroll down until you see the STEP function appear in the list. Click once on the STEP function and click on the Add Sel button. |
| Ø | The argument {height} should be highlighted, just type over it with the value 50. |
| Ø | Double click on the {stime} argument and type in 10. |
| Ø | Enter the units new and click OK. |
